This invention relates generally to computed tomographic (CT) imaging systems, and more particularly to correction of artifacts resulting from beam hardening resulting from high-density materials.
It is known that in the absence of bones or other high-contrast materials, modern CT images produced from scans of patients are artifact-free, mainly due to accurate spectral calibration that removes artifacts for water or soft tissue. However, such images are usually not free of image artifacts resulting from high-density materials.
Image artifacts resulting from high-density materials can be divided into several categories according to their cause. These categories include (1) beam hardening, (2) beam hardening induced detector spectral response variation, and (3) aliasing and photon starvation. Configurations of the present invention are particularly suited for correcting artifacts in the first two categories. Various configurations of the present invention can be used in conjunction with other methods and/or apparatus that correct artifacts in the third category.
The polychromatic nature of x-ray sources used in CT imaging systems induces beam-hardening artifacts in the reconstructed images. In a human body being imaged, there are two main components that lead to distinct beam hardening behaviors: one arising from soft tissue and the other from bone. To complicate this matter, detection efficiency of detector elements changes with x-ray spectrum hardened by different materials, resulting in detection system related image artifacts. In practice, for modern CT scanners, a spectral calibration procedure is performed. This calibration is very effective for water or soft tissue. However, these calibration procedures do not correct for beam hardening problems resulting from other materials. In addition, relative variations in detector response between detector channels is commonly induced by high-density materials, and these calibration procedures are not effective in correcting for these induced variations. For example, head images often require an extra beam hardening correction to remove artifacts for the skull and the complex bone structures inside the head. It is known to correct bone induced beam hardening through a second-pass iteration, where projections due to bones are used to assess and to correct beam hardening in the images. However, the correction for bone induced beam hardening is not effective to correct detector spectral response variations, which tend to create banding or center artifacts in images. As a result, extremely strict requirements are often needed to the guarantee the similarity of detector spectral response behaviors, especially for the detector channels near the isocenter in a third generation CT system.
The purpose of spectral related calibrations in CT imaging systems is to generate a functional form or table that re-maps measured projection values (normalized and minus logged) to its corresponding x-ray path length. The x-ray path length in which the x-rays are hardened can vary, resulting in a variation in the effective attenuation coefficient. This re-mapping ensures that a fixed value attenuation coefficient is formed regardless of the variation in x-ray path length in which the x-rays are hardened. At the same time, detector-related variation is also removed. Known calibrations are performed with only one material, namely, water. Experimental calibration with a mixture of different materials is difficult to perform. Such calibration, were it feasible, would be complex and time consuming.